A property which limits the breadth of application of thermoset polymers and their composites is their relatively low maximum operating temperatures. This work investigates the potential application of both functionalized single-walled carbon nanotubes (f-SWCNTs) based on negative charging, and unfunctionalized SWCNTs (u-SWCNTs) to increase the mechanical and thermal performance of a high-temperature aerospace-grade epoxy with a glass transition temperature of approximately 270 °C. Thermal and mechanical properties of the baseline epoxy and nanocomposites containing a low content of SWCNTs (0.2 % by weight) were characterized through thermogravimetric analyses, tensile tests, and dynamic mechanical analyses. Tensile tests were performed both at room temperature and at 80 °C. Further, room temperature tensile tests were performed on untreated and heat-treated specimens. The heat treatment was performed at 300 °C, slightly above the resin glass transition temperature. Results demonstrate that f-SWCNTs are effective in improving the mechanical and thermal performance of the epoxy. No significant improvement was observed for u-SWCNT nanocomposites. For the nanocomposite with f-SWCNTs, the ultimate tensile strength and strain to failure at room temperature (80 °C) increased by 20 % (8 %) and 71 % (77 %), respectively, as compared to the baseline epoxy. The f-SWCNT nanocomposite, unlike other examined materials, exhibited a stress–strain necking behavior at 80 °C, an indication of increased ductility. After heat treatment, these properties further improved relative to the neat epoxy (160 % increase in ultimate tensile strength and 270 % increase in strain to failure). This work suggests the potential to utilize f-SWCNTs based on negative charging to enhance high-temperature thermoset performance.